The contemporary control technique of using an inverter to drive a motor and control the rotation speed of the motor at the same time is generally classified into scalar control or vector control. Although the scalar control has a poor performance in terms of speed dynamic response, speed ratio control, and the precision control, it has been widely employed in non-server applications as the control architecture of the scalar control is simple, easy, and convergent. The scalar control is also termed as V/f control, or a variable voltage variable frequency control (VVVF). Generally, the scalar control is an open-loop control scheme without the need of the feedback information about the rotation speed of the motor. The basic principle of the scalar control is that the frequency of the power outputted to the motor, i.e. the output frequency of the inverter, is adjusted according to the rotation speed command. As the magnetic flux of the motor is positively proportional to the voltage/frequency ratio of the motor, the output voltage of the inverter has to be adjusted to maintain the ratio of the voltage/frequency of the motor at a constant value, thereby sustaining the magnetic flux of the motor and controlling the rotation speed of the motor.
Although the voltage/frequency control scheme is easy to implement, the error of the output voltage of the inverter under the low frequency and light load condition will aggravate as the output voltage of the inverter is low and the voltage drop of the switches of the inverter is significant. Thus, the control performance of the motor under the low frequency and light load condition is poor. Also, as the power transistors the inverter have non-ideal characteristics such as turn-on delays and turn-off delays, the power transistors will not turn on or off immediately when the input commands are applied thereto. In order to prevent the two transistors on the same rectifier bridge arm to induce a short circuit when the two transistors are not thoroughly turned on or off, a time delay is required between the turn-on time and the turn-off time of the two transistors on the same inverter bridge arm. This time delay is termed as dead time or the time to prevent short circuit.
The AC induction motor generally uses the pulse-width modulation technique to change the amplitude and frequency of the output voltage in order to control the rotation speed of the motor. Due to the architecture of the power transistors, the dead time is necessary to prevent the power transistors from turning on simultaneously during the operation of the inverter. Nonetheless, the dead time may cause a difference between the actual output voltage and the voltage commands. More disadvantageously, the dead time would cause the waveform of the output current to be distorted. Thus, the rotation speed of the motor will be unsmoothed. Thus, it is generally required to carry out open-loop compensation to counterbalance the negative effect as a result of dead time.
FIG. 1A shows the circuit block diagram for performing dead-time compensation in the inverter according to the prior art, which is by far the most common dead-time compensation scheme. As shown, the dead-time compensation scheme for the inverter 10 is accomplished by detecting the three-phase current of a motor 11 to calculate the required dead-time compensation quantity. That is, a current detector 12 is used to detect the input current of the motor 11, i.e. the current detector 12 is used to detect the three-phase output current of the inverter 10. The three-phase output current of the inverter 10 is received by a dead-time compensation module 13. According to the polarity of the three-phase output current, a pulse-width modulation (PWM) voltage command adds or subtracts an offset voltage to produce a trapezoid compensation curve in phase with the output current. Such dead-time compensation scheme is advantageous in terms of simple calculations. However, such dead-time compensation scheme has the advantage that the voltage compensation quantity and the slope of the trapezoid compensation curve will be deviant from the ideal value, which would in turn distort the output current waveform. Hence, the rotation speed of the motor will be discontinuous. Such distortion will be more obvious under the low-frequency and light load condition.
In order to address the aforementioned problem of the distorted current waveform of the output current induced when the motor is operating under the low frequency and light load condition, another dead-time compensation scheme has been proposed. FIG. 1B shows another kind of the circuit block diagram for performing dead-time compensation in the inverter according to the prior art. The dead-time compensation scheme shown in FIG. 1B is accomplished by way of voltage feedback. Compared to FIG. 1A, the circuitry of FIG. 1B additionally includes a voltage detector 14. The voltage detector 14 is used to detect the three-phase output voltage of the inverter 10 and obtain the instantaneous output voltage drop. The instantaneous output voltage drop and the polarity of the detected three-phase current are used to calculate the voltage compensation quantity and direction. By using such dead-time compensation scheme, the waveform of the output current is a smooth compensated curve being approximate to a sinusoidal wave. As discussed above, the compensation curve of the dead-time compensation scheme of FIG. 1A is a trapezoid. By using such dead-time compensation scheme of FIG. 1A, the current waveform will be seriously distorted under a high output voltage condition, and the voltage compensation quantity will be too much as the compensation quantity of the trapezoid compensation curve is inconsistent with the actual compensation quantity. By using such dead-time compensation scheme of FIG. 1B, a compensation quantity with high accuracy and an undistorted sinusoidal current are obtained, thereby suppressing the distortion of the output current waveform induced when the motor is operating under the low frequency and light load condition. However, as the dead-time compensation scheme of FIG. 1B requires an additional voltage detector 14, the cost of hardware circuitry is elevated.
In summary, the conventional dead-time compensation schemes uses the feature that the output voltage is different from the actual voltage command to counterbalance the negative effect of the dead time, thereby smoothing the waveform of the output current to allow the waveform of the output current to be approximate to a sinusoidal wave. However, these conventional dead-time compensation schemes will cause the voltage compensation quantity and the slope of the trapezoid compensation curve to deviate from the ideal value. Thus, the waveform of the output current is distorted. In this manner, the rotation speed of the motor will be non-uniform, which means that the rotation speed of the motor will alternate between slow and fast. The cost of hardware circuitry of the inverter will increase accordingly.
It is needed to develop a driver having dead-time compensation function with low cost and excellent insusceptibility to frequency fluctuation. The invention can meet these needs.